Non-volatile memory devices are used in a multitude of everyday electronics, e.g. smartphones, music players, USB-sticks, memory cards, e.g. for digital cameras, solid-state disks (SSDs) etc.
Non-volatile memories such as the so-called EPROM-technology has certain limits as regards the storage density, access, erase and writing times making it undesirable for the constantly increasing data volume in modern applications, such as the ones mentioned above.
A further technology for non-volatile memories are resistive switching memories which are formed of arrays of resistive switching elements. Each of these elements has two or more stable resistive states. Switching between the states is accomplished by specific voltage pulses.
Resistive switching elements use a “forming process” to prepare a memory device for use. The forming process is typically applied at the factory, at assembly, or at initial system configuration. A resistive switching material is normally insulating, but a sufficient voltage (known as a forming voltage) applied to the resistive switching material will form a conductive pathway in the resistive switching material. Through the appropriate application of various voltages (e.g. a set voltage and reset voltage), the conductive pathways may be modified to form a high resistance state or a low resistance state. For example, a resistive switching material may change from a first resistivity to a second resistivity upon the application of a set voltage, and from the second resistivity back to the first resistivity upon the application of a reset voltage which voltages are usually different from each other.
Two types of ReRAM switching elements are currently under investigation, namely valence change memory (VCM) and electrochemical metallization memory (ECM). In VCMs oxygen anions are removed from a metal oxide matrix whereupon the conductivity of the metal oxide matrix increases. In ECMs metal ions are reduced and build filaments within the matrix between the two electrodes thereby increasing the electrical conductivity. It has recently been found (cf. http://www.fz-juelich.de/SharedDocs/Pressemitteilungen/UK/DE/2015/15-09-28nnano_reram.html) that in VCMs in addition to the oxygen ion movement also filaments are formed. However, in VCMs elemental oxygen needs to be stored which may migrate to the surface of the electrode and cause delamination thereof.
In current solid state electrolytes usually amorphous metal oxides are used as a matrix whereby the transport of a metal ion therethrough leads to built-up and dissolution of a metallic filament between the two electrodes attached thereto.
After a multitude of switching processes the structure of the solid state electrolyte changes leading to separate metallic phases of the ion to be transported and, in turn continuously changes the switching behavior of the electrolyte making the switch unusable as memory. Moreover, a high degree of purity is desired in order to suppress side reactions and phase separations.
Thus, a resistive switching memory cell is needed which is not prone to changes in the switching behavior or which at least has a significant lower rate of change thereof due to the charge-state stabilization of the mobile ion in the matrix.